As health and environmental issues become more prominent, government agencies have reacted by promulgating mandates to prevent the escape of volatile hydrocarbon vapors to the atmosphere. Industry has repeatedly focused its attention on curtailing hydrocarbon vapor release from vehicular fuel tanks containing gasoline or other highly volatile hydrocarbon fuels. The industry has responded to this danger by employing an on-board Evaporative Emission Control (EVAP) System to prevent vapor escape. Briefly, an EVAP system includes a canister, containing activated carbon pellets, connected by vapor flow lines to the fuel tank and to the vehicle's intake manifold. As vapor is formed in the fuel tank, it can flow to the canister, where vapor is adsorbed by the pellets. Periodically, a purge cycle sends accumulated vapor from the canister to the intake manifold to be combusted.
An EVAP system does not offer perfect vapor control, however. For example, a canister may become saturated, which can lead to vapor escape as excess vapor is generated. Fuel refill events can pose a particular problem, as incoming fuel forces existing vapor in the fuel tank into the canister, where the surge of vapor can swamp the canister's capacity. Thus, EVAP systems by themselves may not be the entire answer to vapor emission problems.
Conventional responses to the risk of vapor emission have focused on increasing the size of the canister. That solution, however, increases direct cost, in terms of the canister itself, but the larger canister requires further modifications to the automotive design, further increasing costs.
As vapor release remains a substantial risk, controlling fuel vapor emissions effectively remains a priority in modern vehicles.